1. Field of the Invention
The present invention relates to molded polyurethane/urea elastomers. Specifically, the invention relates to improved molded polyurethane/urea elastomers with high and low temperature resistance in dynamic applications, and especially to improved belts utilizing these improved elastomers. These polymeric elastomers exhibit improved thermal stability while maintaining acceptable static and dynamic properties, including chemical resistance, cold flexibility, flex crack resistance and low hysteresis.
2. Description of the Prior Art
Belts, such as timing or synchronous belts, V-belts, multi V-ribbed belts, micro-ribbed belts, flat belting and the like, constructed from polyurethane or polyurethane/urea elastomers offer a number of significant advantages over conventional rubber belting. Among these advantages are that polyurethane and polyurethane/urea belts have greater flex resistance, oil resistance, hydrolysis resistance, and demonstrate improved load carrying capability. These belts can be vacuum spin cast in a single operation, injection molded, or batch cast. Conventional rubber tooth-type belts require numerous fabrication steps.
Polyurethane/urea based elastomers are traditionally prepared by reacting a relatively high molecular weight active hydroxyl-terminated material, such as a polyol, and a relatively low molecular weight active amine-terminated material, known as a chain extender, with a polyisocyanate via either one-shot or two-step (prepolymer) approach. In preparing the elastomer, the reactive components and any catalysts or optional additives are blended together and then transferred to a mold of suitable shape where the formulation is cured. Typically, this blending is accomplished in a batch process. The mixture is cured in the mold until it is capable of maintaining the molded shape, demolded and postcured until polymerization is complete. Alternatively, the elastomer may be prepared via Reaction Injection Molding (RIM), in which the active hydrogen containing materials are mixed rapidly with polyisocyanate via impingement and simultaneously injected into a mold where the reaction takes place.
Relatively high and low temperature resistant molded belts utilizing polyurethane/urea elastomers which are reaction products of 4,4′-diphenyl methane diisocyanate (MDI), poly(propylene oxide) polyol, and a diamine chain extender are known. These elastomer formulations typically include an antioxidant to improve thermal stability. The antioxidant is necessary to retard the oxidation of polyether polyols, which normally takes place at 100–130° C. Notably, the antioxidant retards but does not eliminate polyether oxidation at high temperatures, thus the thermal stability initially observed diminishes with increased time and usage of the belt. Additionally, the formulations typically allow for the use of catalysts which are known to accelerate reversion of the resulting elastomers at high temperatures.
An improvement in the thermal stability of polyurethane and polyurethane/urea elastomers is achieved by utilizing para-phenylene diisocyanate (PPDI) in the polyisocyanate prepolymer composition. The high isocyanate reactivity differential of PPDI relative to MDI results in a decrease in oligomer formation and a proportionate decrease in free diisocyanate in the prepolymer. Consequently, PPDI prepolymer promotes greater phase separation of the hard and soft segments, and hence better thermal stability of the resulting elastomer.
Thermoplastic polyurethane resin prepared from PPDI, a poly(hexamethylene carbonate) polyol, and a short chain polyol such as 1,4-butanediol or 1,4-bis-(beta-hydroxyethoxy) benzene is known. A disadvantage of this material is the use of hydroxyl-terminated chain extenders in forming the resulting polyurethane elastomer. While exhibiting favorable processing characteristics, it has been found that hydroxyl-terminated chain extenders form relatively weak urethane linkages with the polyisocyanate prepolymer, which detract from polyurethane elastomer's performance in dynamic applications.
Various factors contribute to traditional polyurethane or polyrethane/urea elastomer's thermal instability. These include polyether oxidation if polyether polyol is used as the soft segment, rapid reversion if catalysts are present in the formulations, poor phase separation if polyisocyanates or chain extenders with unfavorable structures are used, and relatively weak urethane linkages if hydroxyl chain extenders are used. In certain belt applications such as automotive timing or synchronous belts, V-belts, multi V-ribbed belts, micro-ribbed belts, and the like, such belts are subjected to repeated high and low temperature extremes in dynamic loading conditions. Polyurethane and polyurethane/urea elastomers have to date been unacceptable for these long term dynamic applications due to their tendency to shear and/or crack under these conditions. The difficulties become more apparent as the demand for automotive engines with higher application temperatures increases. Thus, there remains a need for polyurethane/urea elastomer incorporated in belts that have excellent load carrying capability as well as the characteristics necessary to withstand repeated dynamic loading under high and low temperature conditions for long periods of time, whether such belts are in the form of timing or synchronous belts, V-belts, multi V-ribbed belts, micro-ribbed belts, or the like.